1. Field of the Invention
The present invention relates to a solid-state imaging device, and more particularly to a charge coupled device (CCD) type of solid-state imaging device comprising a two-level charge transfer electrode structure.
2. Description of the Related Art
In recent years due to the popularization of a compact video camera, there has been increased demand for solid-state imaging devices, especially CCD type solid-state imaging devices.
A conventional CCD type solid-state imaging device comprises a plurality of first charge transfer electrodes 301 and a plurality of second charge transfer electrodes 302 which are extended in a horizontal direction on the top surface of a silicon substrate as shown in FIG. 3. In FIG. 3, the first charge transfer electrodes 301 are shown by broken lines. The first charge transfer electrodes 301 and the second charge transfer electrodes 302 are each arranged in a vertically separated pattern at a prescribed separation distance. Thus, the conventional solid-state imaging device has a two-level electrode structure in which the first charge transfer electrodes 301 function as lower electrodes, and the second charge transfer electrodes 302 function as upper electrodes. Each first charge transfer electrode 301 comprises a first connecting section 311 and a first gate section 312, the first connecting section 311 extending linearly in the horizontal direction, the first gate section 312 projecting in the vertical direction at one side of the first connecting section 311. On the other hand, each second charge transfer electrode 302 comprises a second connecting section 321 and a second gate section 322, the second connecting section 321 extending linearly in the horizontal direction so as to almost overlap the first connecting section 311, the second gate section 322 projecting in the vertical direction at one side of the second connecting section 321 so as to be symmetrical with the first gate section 312.
The first and second gate sections 312 and 322 of the adjoining first and second charge transfer electrodes 301 and 302 are overlapped with each other at end portions thereof so as to successively cover charge transfer sections 305 along the vertical direction. In FIG. 3, an arrow in the charge transfer section 305 shows a transfer direction of signal charge. A photoelectric converter 303 is provided at a region between the first and second charge transfer electrodes 301 and 302, i.e., a rectangular region bounded by the edges of the first and second connecting sections 311 and 321 and the first and second gate sections 312 and 322. The charge transfer section 305 and the photoelectric converter 303 constitute an oblong unit pixel. An arrow 306 shows a transfer point of the signal charge from the photoelectric converter 303 to the associated charge transfer section 305.
On the photoelectric converter 303, a microlens 308 may be provided so as to improve a condensing efficiency of incident light as shown in FIG. 4. A one-dotted chain line 307 shows an array of unit cells of the microlens 308. The microlens 308 is actually provided over the photoelectric converter 303 via a transparent planarized layer. The microlens 308 is made as follows: First, a photopolymer layer based upon thermoplastic resin is provided on a planarizing polymer layer, etc. on the semiconductor substrate on which the charge transfer section 305 and the photoelectric converter 303 are formed. Then the photopolymer layer is patterned into a plurality of rectangular shapes using photolithographic techniques, so that each of the rectangular shapes corresponds to a pixel. Finally, each rectangular shape is thermally fused so that the corner portions thereof are made round.
Generally, the solid-state imaging device has been improved in the three areas of high integration, high resolution, and high sensitivity. However, in the above conventional solid-state imaging device, there are the following problems which impede further improvement of solid-state imaging devices.
First, there is a problem in that the withstand voltage property of the device is decreased as the device is highly integrated. That is, if the first and second charge transfer electrodes 301 and 302 are maintained in their shapes as shown in FIG. 3, pixels cannot be integrated unless the unit pixel cell is reduced in size. In a case where the unit pixel cell is reduced in size, each line width (in the horizontal direction) of the first and second gate sections 312 and 322 is narrowed, and consequently electric field concentration will occur at end portions of the first and second gate sections 312 and 322. A layer insulating film is interposed at the overlapped portion of the end portions of the first and second gate sections 312 and 322; at a portion between the first gate section 312 and a base silicon active layer; and at a portion between the second gate section 322 and the base silicon active layer. However, in a case where high electric field concentration occurs due to high integration, insulation breakdown and short circuits will inevitably occur between the layers.
It is difficult to decrease a transfer pulse voltage when the device is highly integrated. On the contrary, an inversion threshold voltage is increased due to narrow channel effects as the device is highly integrated, and therefore the transfer pulse voltage must be increased. Thus, to highly integrating the solid-state imaging device (i.e., for reducing the size of a chip), the withstand voltage of the device should be increased.
Thus, the conventional CCD type imaging device is designed considering the withstand voltage thereof. In order to avoid insulation break down, the interlayer insulating film should not be made thinner, and thus it becomes difficult to reduce the dimension of the device in a direction perpendicular to the top surface of the substrate (i.e., a depth direction). As the device is highly integrated, an aspect ratio (i.e., a dimension in the depth direction/a dimension in the horizontal direction) is increased, so that part of the light is blocked due to an electrode or the like in the vicinity of a light receiving section. As a result, it becomes extremely difficult to condense incident light onto the light receiving section with the microlens.
Secondly, there arises a problem in that sensitivity of the device is lowered and charge storing capacity of the light receiving section is decreased as the device is highly integrated. The photo-sensitivity of the photoelectric converter 303 shown in FIG. 3 is almost proportional to the area of the light receiving section. Accordingly, in a case where the pattern of the top surface of the device is simply and proportionally reduced while maintaining the first and second charge transfer electrodes 301 and 302 in their shapes, the area of the light receiving section is reduced, and consequently a necessary amount of signal charge cannot be ensured. The proportional reduction of the pattern leads to high resolution. Therefore, in order to ensure a sufficient amount of signal charge, it is necessary to enlarge the area of the light receiving section as much as possible.
Thirdly, there arises a problem in that there are many non-effective areas 309 which do not contribute to condensing of incident light in a case where the microlens 308 is disposed so as to have an oblong shape, since the corner portions of the thermoplastic polymer are contracted to be round. This problem becomes further noticeable in a case where a unit pixel cell 307 is reduced in its size for higher integration. The reason is that the balance of the thermal fusion and the surface tension of the material for the microlens 308 becomes worse as the pattern is reduced in its size, and finally the shape of the microlens 308 becomes almost hemispherical. Therefore, the total area of the non-effective areas 309 should be reduced in order to efficiently condense the incident light onto the photoelectric converter 303.